() => {
    //函数的可选参数
    const fn1 = (a, b) => { };
    //函数参数的默认值
    const fn2 = (a, b = "1") => { };
    //剩余参数
    const fn3 = (a, b, ...arg) => { };
    function fn4(a, b) {
        if (typeof a === "number" && typeof b === "number") {
            return a + b;
        }
        if (typeof a === "string" && typeof b === "string") {
            return a + b;
        }
    }
    // fn4(1, "2")
    fn4(1, 2);
    fn4("1", "2");
};
